1. Field of the Invention
The present invention relates to a photodetector included to be used in an optical pickup apparatus performing an operation for reading signals recorded in an optical disc and an operation for recording signals into an optical disc and the optical pickup apparatus including the photodetector.
2. Description of the Related Art
Optical disc apparatuses are widely used, which apply laser beams emitted from optical pickup apparatuses to signal recording layers of optical discs to perform signal reading operations and signal recording operations.
While are generally used optical disc apparatuses using optical discs called CD or DVD, there are recently developed optical disc apparatuses using optical discs having improved recording densities, i.e., optical discs of the Blu-ray standard or HD DVD (High-Definition Digital Versatile Disc) standard.
Infrared light with a wavelength of 780 nm is used as the laser beam for the operation of reading signals recorded in a CD-standard optical disc, and red light with a wavelength of 650 nm is used as the laser beam for the operation of reading signals recorded in a DVD-standard optical disc.
The laser beam with a shorter wave length, for example, blue-violet light with a wavelength of 405 nm is used as the laser beam for the operation of reading signals recorded in the Blu-ray standard or HD-DVD standard optical discs, as compared to the laser beam for the operation of reading signals recorded in the CD-standard or DVD-standard optical discs.
A protective layer disposed on a top surface of the signal recording layer of the Blu-ray standard optical discs has a thickness of 0.1 mm, and a numerical aperture of an objective lens used for the signal reading operation from the signal recording layer is set to the specified value of 0.85.
On the other hand, a protective layer disposed on the top surface of the signal recording layer of the HD-DVD standard optical discs has a thickness of 0.6 mm, and a numerical aperture of the objective lens used for the signal reading operation from the signal recording layer is set to the specified value of 0.65.
Since the blue-violet light with a wavelength of 405 nm can be used as the laser beam for the operation of reading signals recorded in the Blu-ray standard or HD-DVD standard optical discs, as above, there can be produced an optical pickup apparatus that can perform the operation of reading signals from optical discs of both standards by sharing a laser diode.
The optical pickup apparatus is configured such that a drive current to be supplied to the laser diode can be controlled to obtain an laser output suitable for reading signals recorded in the optical disc or to obtain the laser output suitable for recording signals into the optical disc.
The optical pickup apparatus is configured so as to be able to perform a control operation for focusing the laser beam emitted from the optical pickup apparatus, as a spot, onto the signal recording layer of the optical disc, i.e., a focusing control operation, and performing a control operation for causing the spot of the laser beam to follow a signal track, i.e., a tracking control operation.
In the optical pickup apparatus, a photodetector is included as a means for performing the laser output control operation, the focusing control operation, and the tracking control operation as described above. The photodetector is generally configured such that a light-receiving element such as a photodiode is covered with a translucent resin (see Japanese Patent Application Laid-Open Publication No. Sho 63-000830).
The photodetector included in the optical pickup apparatus will be described with reference to FIGS. 7 and 8. FIGS. 7 and 8 are a cross-sectional view and a plain view, respectively, of the photodetector. A photodiode 2 for outputting a signal according to a light amount of a received laser beam is fixed on a substrate 1, and the photodiode 2 is covered with a translucent resin 3 having high transmittance for laser beam having a wavelength to be used. Signals generated by the photodiode 2 are picked up through bonded wires, and such wires are fixed and protected generally by the translucent resin 3.
While the laser be amused for the DVD optical discs, i.e., a red laser beam, has a wavelength of 650 nm, the laser beam used for the Blu-ray standard and HD-DVD standard optical discs, i.e., a blue-violet laser beam, has a shorter wavelength of 405 nm.
A relationship between energy and wavelength of a laser beam can be represented by E=hν=h×c/λ, where E is energy; h is Planck's constant; ν is frequency; c is velocity of light; and λ is wavelength. From this relational expression, such a characteristic is clear that light energy is increased as a laser-beam wavelength is reduced.
In the optical pickup apparatus including a laser diode emitting a laser beam, the laser output control operation, the focus control operation, etc., are performed with using a signal obtained from the photodetector, and the laser-beam light amount detection operation by the photodetector is performed based on the laser beam applied through the translucent resin 3 covering the photodiode 2.
Although an epoxy resin is generally used as a material of the translucent resin 3 in consideration of performance and cost, when using resin, transmittance is deteriorated due to a photochemical reaction, and thus, unfortunately the laser-beam light amount detection operation cannot be performed with accuracy.
Although a method of forming a through-hole 3a on an laser-beam incident surface of the photodiode 2 as shown in FIGS. 7 and 8 is employed as a method of solving the above problem, since the hole is required to be formed with accuracy in an area through which the beam of laser light passes, a forming processing unit cost is increased, which is a problem.
In order to reduce the forming processing unit cost, there is employed a photodetector having a configuration without the through-hole 3a formed on the laser-beam incident surface by using resin with higher transmittance as the translucent resin.
The translucent resin covering the photodiode has a property that transmission characteristics are changed corresponding to a wavelength of a laser beam to be used. For example, if a synthetic resin material used for an objective lens for focusing the laser beam corresponding to the DVD standard, i.e., red laser beam with a wavelength of 650 nm, is used to produce an objective lens for focusing the laser beam corresponding to the Blu-ray standard and the HD-DVD standard, i.e., the blue-violet laser beam with a wavelength of 405 nm, the transmittance of the objective lens is considerably reduced by the blue-violet laser beam.
FIG. 6 is a characteristic view of a change in the transmittance of the translucent resin for the blue-violet laser beam with time in the case of using the synthetic resin material having characteristics suitable for the red laser beam such as the synthetic resin material called APEL 5014DP of Mitsui Chemicals, inc., as the translucent resin. It is experimentally confirmed that the transmittance is reduced by 2% or more in the case where the blue-violet laser beam with a wavelength of 405 nm is applied for 1000 hours. If the transmittance of the translucent resin covering the photodiode is reduced as above, the intensity of the laser beam applied to the photodiode is reduced, and thus, detection characteristics as a photodetector are deteriorated.
The translucent resin is specified to be set, as an usable resin standard, such that the degree of change in transmittance is within a range of 2% in the case where the laser light is applied thereto for 1000 hours, for example, and if the degree of change in the transmittance is out of specification, a level of an output signal corresponding to intensity of the laser light applied to the photodetector becomes out of permissible range.
In solving such a problem, there is a method of using the synthetic resin which is not affected by the blue-violet laser beam, such as synthetic resin called ZEONEX 340R of Zeon Corporation as a material of the translucent resin covering the photodiode. However, such synthetic resin is expensive as compared to the synthetic resin material generally used for the photodetector for DVD, and therefore, unfortunately a price of an optical pickup apparatus cannot be reduced.